Transmission/reception unit with bidirectional equalization

ABSTRACT

The present invention refers to a transmission/reception unit that exchanges data with a remote unit by means of a channel using distributed modulation/demodulation treatment means. The transmission/reception unit includes equalization means ( 23 ) to equalize a first signal received from said remote unit and predistortion means to predistort a second signal transmitted to said remote unit. According to the invention, the unit is characterized in that the modulation/demodulation treatment means operate in a non-linear way and the unit includes means ( 230 ) to store information that is representative of a non-linear distortion of said modulation/demodulation treatment means and which defines a non-linear reference, and means ( 233 ) to adjust coefficients (CO) in the equalization means ( 23 ) according to the signal received from the remote unit and said representative information, the mentioned adjusted coefficients being transmitted to said predistortion means.

OBJECT OF THE INVENTION

[0001] The present invention refers to a data transmission/receptionunit to exchange change data with a remote unit by means of a channelusing non-linear modulation/demodulation treatment means, for examplefrequency modulation/demodulation.

STATE OF THE ART

[0002] The use of frequency modulation methods, like that used in theEuropean standard for wireless communication DECT, of the GFSK type,makes the use of frequency demodulators, for example by means ofdiscriminators, recommendable; in this way, the recovery of data ispossible by means of a simple threshold detector to decide between thetwo logic values “1” or “0”.

[0003] In some fields of application of these methods, this type ofreceiver is sufficient due to the fact that the maximum temporarydispersion of the signal in the receiver is rather less than a symboland, hence, the interference between symbols due to the radio channel ishardly detectable.

[0004] However, in other applications where radio coverage is greaterand induces multiple trajectories of the waves, the effects ofattenuation and reflection in the propagation thereof have to be takeninto account. Therefore, the use of equalizers is recommendable, whichrectify this effect that degrades the signal quality, considerablyenhancing the probability of error of the recovered symbols. For thisreason, some kind of equalizer is used, as shown in the article“Adaptive equalization for DECT systems operating in low time-dispersivechannels” by J. Fuhl and G. Schultes, published in the magazine“Electronics Letters”, Nov. 25, 1993, vol. 29, N 24, pages 2076-2077. Inthis article, an adaptive equalizer of the well-known DFE (decisionfeedback equalizer) type is used and where 16 bits of burstsynchronization defined in the DECT are used as a training sequence. Thetraining sequence is stored in a ROM memory and is applied to theequalizer when the signal corresponding to the training sequencereceived appears in the input thereof. This avoids that the possibleerrors that are produced in the detector when the coefficients of thefilters are still unsuitable are fed back to the equalizer and permitsthat the convergence is produced faster.

[0005] When the type of modulation/demodulation is non-linear, evenunder ideal propagation conditions the demodulated signal may beconsiderably distorted due to the mentioned non-linear effect. Underthese conditions, the convergence of the equalizer may be inadequatelyproduced, since the non-linear process to which the received signal issubmitted is not being duly taken into account. For example, thefrequency modulator and the frequency discriminator introduce non-lineardistortions given that the voltage-frequency conversion characteristicon the one hand, and the frequency-voltage on the other, are not aconstant as in the ideal case.

[0006] More specifically, the invention refers to atransmission/reception unit, including a bidirectional correction devicefor the inter-symbolic interference acting through both directions ofthe radio channel. The device permits the correction of interferences inboth directions of the data route, including an equalizer in thereception chain and a predistorter in the transmission chain. Theobjective of incorporating the bidirectional correction function of theinter-symbolic interference in the same transmission/reception unit isto reduce the cost of the other unit(s) that exchange data with saidunit.

[0007] A device of this type, including an equalizer and a predistorter,is known from the document “Adaptive channel precoding for personalcommunications” by W. ZHUANG et al., published in the magazine“Electronics Letters”, Sep. 15, 1994, vol. 30, N 19, pages 1570-1571. Inthis document, a solution to the problem of linearmodulation/demodulation is proposed, this being phase modulation.

[0008] This limitation is the result of the superposition of thenon-linear effects and the inter-symbolic interference effects due tothe multiple trajectories of the waves, which hinders a “bidirectionalequalization” in only one transmission/reception unit. If the non-lineareffects do not permit the effective correction of the inter-symbolicinterference in the received signal, an imperfect predistortion results,which may be worse than not using this predistortion. Furthermore, therequirements for updating the equalizer coefficients and thepredistorter coefficients for each transmission/reception period using ashort signal sequence to assess the channel status have to be taken intoaccount.

CHARACTERIZATION OF THE INVENTION

[0009] One objective of the present invention is to propose atransmission/reception unit for a communications system operating in anon-linear mode, that includes a bidirectional correction device for theinter-symbolic interference. The invention is applied to the DECTsystem, but not exclusively.

[0010] As a result, a data transmission/reception unit that exchangesdata with a remote unit by means of a channel using distributedmodulation/demodulation treatment means distributed between said datatransmission/reception unit and said remote unit, saidtransmission/reception unit including equalization means to equalize afirst signal received from said remote unit and predistortion means topredistort a second signal transmitted to said remote unit ischaracterized according to the invention in that (a)—themodulation/demodulation treatment means work according to a non-linearmode and (b)—the unit includes

[0011] (b1) means to store information which is representative of anon-linear distortion of said modulation/demodulation treatment meansand which defines a non-linear reference and

[0012] (b2)—means to adjust equalization means coefficients according tothe signal received from the remote unit and said representativeinformation, the mentioned adjusted coefficients being transmitted tosaid predistortion means.

[0013] Therefore, both in the equalizer and in the predistorter, thecorrection of the inter-symbolic interference is carried outindependently from the non-linear distortion resulting frommodulation/demodulation.

[0014] According to a first embodiment, the transmission/reception unitis characterized in that the adjustment means are calculation meansminimizing an error signal in the equalization means, on the one handaccording to said signal received from the remote unit and, on theother, to the mentioned representative information.

[0015] According to a second embodiment, the adjustment means include:

[0016] means to sample said first signal received from the remote unitto a frequency which is higher than a frequency of logic symbolsassociated to the received signals and to define sets of samples, eachone defined by a sample phase,

[0017] means to correlate said representative information with each oneof the sample sets of the signal received from the remote unit, in orderto determine an a priori optimum sample set,

[0018] means to select a sample set between said a priori optimum sampleset and some of said sample sets defined by respective sample phasesnear to the sample phase of said a priori optimum sample set, theselected set being the one that produces the smallest equalization errorsignal with respect to a sequence of logic symbols, and

[0019] calculation means to minimize an error signal in the equalizationmeans according, on the one hand, to said signal received from theremote unit when it is sampled with the sample phase of the selectedsample set, and on the other hand, to said logic symbol sequence.

BRIEF DESCRIPTION OF THE FIGURES

[0020] A more detailed explanation of this invention is given in thefollowing description based on the attached figures, in which:

[0021]FIG. 1 shows a block diagram of a transmission/reception unitoperating according to a phase predistortion mode;

[0022]FIG. 2 shows a block diagram of a transmission/reception unitoperating according to a predistortion mode for components inquadrature;

[0023]FIG. 3 shows a block diagram of an equalizer according to thestate of the art;

[0024]FIG. 4 shows a block diagram of one part of the reception chain ofthe transmission/reception unit according to the preferred embodiment inaccordance with the invention; and

[0025]FIG. 5 shows an algorithm of a second embodiment according to theinvention.

DESCRIPTION OF THE INVENTION

[0026] The present invention, in a preferred embodiment, is applied in abase station for a digital communications system with GFSK modulationused in the European digital system of wireless telecommunications(DECT). This base station exchanges data with a remote unit, for examplea terminal, using a duplex mode by division in time.

[0027] In FIG. 1, the base station includes a transmission chain and areception chain. The transmission chain includes, connected in cascade,a source of binary data 10, a Gauss filter 11, an integrator 12, apredistorter 13, two circuits 14 a and 14 b for separation of thecomponents in quadrature, a low pass filter 15 and an elevator converter16, the output of which is connected to an antenna. The reception chainincludes, connected in cascade, an RF filter 20, a frequency reducer 21,a limiter/discriminator 22, an equalizer 23 and a digital system 24. TheGauss filter 11 and the integrator 12 are used to modulate the binarydata to be sent in frequency and the limiter/discriminator 22 is used todemodulate the signal received in frequency.

[0028]FIG. 2 shows the predistorter including two predistortion circuits13 a and 13 b. Each one predistorts a signal corresponding to acomponent in-phase and in quadrature. In this embodiment, the integrator12 output is applied to the two circuits 14 a and 14 b for separation ofthe components in phase and in quadrature. The outputs of these twocircuits 14 a and 14 b are respectively applied to the two predistortioncircuits 13 a and 13 b. In FIG. 1, a coefficient transfer output of theequalizer 23 is applied to a coefficient updating input of thepredistorter 13. In FIG. 2, a coefficient transfer output of theequalizer 23 is applied to a coefficient update input of each one of thetwo predistortion circuits 13 a and 13 b.

[0029] In a third embodiment (not shown), the reception chain includes,connected in cascade, the digital data source, a predistorter, a Gaussfilter, a VCO and an elevator converter.

[0030] The predistorter 13 (FIG. 1) or the predistortion circuits 13 aand 13 b (FIG. 2) modify the signal received in their inputs with atransfer function compensating the inter-symbolic interference due tothe radio channel.

[0031]FIGS. 3 and 4 show a first preferred embodiment of the invention.FIG. 4 shows the limiter/discriminator 22 of the reception chain of thebase station including an amplitude limiter 220, a discriminator 221 anda low pass filter 222.

[0032] The function of the limiter 220 is to trim the signal, so that atits output the amplitude of the signal is always constant. This allowsits demodulation to be performed without having to resort to automaticgain control methods. On being a system with frequency modulation andconstant envelope, when filtering the trimmed signal again with anintermediate frequency filter, we obtain a signal with the sameinformation as that received but this time with an always constantamplitude.

[0033] The frequency discriminator 221 is normally a multiplier which isa non-linear component, multiplying the signal to be demodulated byanother which is that derived from the former and besides dephased 90°.The output signal of the discriminator 221 is sent to the low passfilter 222 which eliminates noise, especially that of higher frequencieswhose effect is more damaging, since as we know, the spectral noisedensity at the output of a frequency demodulator is of the parabolictype.

[0034] Finally, the output signal of the aforementioned low pass filter222 is sent, by means of the equalizer 23, to a symbol detector 24 athat belongs to the digital system 24. This detector 24 a is, in thesimplest case, a comparator with a reference level corresponding to thatprovided by the demodulator in absence of a modulated signal; that is,when only the carrier is received. In this circumstance, the valuesabove the reference level are equivalent to a logic value and the valuesbelow are equivalent to another logic value. Obviously, much morecomplex detectors can be used which obtain a lower probability of errorfor the same noise conditions, but this does not affect the object ofthe invention.

[0035] Through the remote terminal transmission chain up to the input tothe symbol. detector 24 a of the base station, the signal passes throughdifferent components distorting the signal non-linearly, which are notin principle taken into account when equalizing and predistortingaccording to the state of the art described in the document “Adaptivechannel preceding for personal communications” by W. ZHUANG et al.,published in the magazine “Electronics Letters”, Sep. 15, 1994, vol 30,N 19, pages 1570-1571.

[0036] For this reason, according to the invention, the equalizer 23includes a wave shaper 230 which receives symbols of a training sequenceTS formed of bits with logic states “1” and “0”. This sequence islocally stored in the base station and is identical to a sequence sentby the remote terminal. The wave shaper 230 stores information which isrepresentative of a non-linear distortion of the non-linearmodulation/demodulation means which are distributed between the basestation and the remote terminal or unit. This information defines anon-linear reference. The components taken into account in the transferfunction of the wave shaper 230 are, for example, in the remote terminaltransmission part, the premodulation Gauss filter and the frequencymodulator and in the reception part of the base station, the amplitudelimiter 220, the frequency discriminator 221 and the low pass filter222. The specialist can limit the transfer function of the wave shaper230 to some of these chain components of the terminal transmission chainand the reception chain of the base station which induce non-lineareffects. The training sequence TS, which in this case for DECT is theburst training sequence, is stored in a logic circuit 231. For each DECTburst, an equalizer training phase is produced. This training phase isproduced when the training sequence transmitted by the remote terminalis received in an input of a first FFF filter of the equalizer 23 of thebase station. In this moment, the stored TS sequence is applied to aninput of a second FBF filter by means of the wave shaper 230 by a switch232.

[0037] As shown in FIGS. 3 and 4, the equalizer 23 includes, accordingto a known embodiment, the linear filter with feed FFF (Feed ForwardFilter) and the linear filter with feedback FBF (FeedBack Filter), bothdefined by a delay “T” and coefficients CO which are c⁻², c⁻¹, c₀, c₁and c₂ and also a calculation module 233. According to the invention,the equalizer 23 also includes the logic circuit 231 which stores theburst synchronism sequence TS and the wave shaper 230 whose input isconnected to an output of the logic circuit 231. An FFF filter output isconnected to an input of the symbol detector 24 a, through an adderwhose second input is connected to an FBF filter output. During thetraining phase, the output of the wave shaper 230 is applied to an FBFfilter input. Except during this phase, the output of the symboldetector 24 a is applied to an FBF filter input.

[0038] During the equalizer training phase, an algorithm stored in thecalculation module 233 changes the CO coefficients=(c⁻², c⁻¹, c₀, c₁ andc₂) of the FFF and FBF filters, so that the signal produced by the waveshaper 230, when it receives the training sequence TS stored in a logiccircuit, is as similar as possible to the signal produced at the symboldetector input, so that an error signal ES is obtained between these twosignals whose mean square value asymptotically tends to zero.

[0039] At the end of the equalizer 23 training phase, the adaptationalgorithm developed in the calculation module 233 is frozen so that thecoefficients c⁻², c⁻¹, c₀, c₁ and c₂ calculated in both FFF and FBFfilters of the equalizer 23 are maintained constant until the end of theburst or raster. According to the invention, the coefficients obtainedat the end of the training phase are transmitted to the predistorter 13(FIG. 1) or 13 a and 13 b (FIG. 2). The same coefficients are used toequalize the data received during a reception time interval whichtemporarily follows the sequence TS time interval, includes data andpredistorts the signal transmitted through the transmission chain of thebase station. The predistorter includes the same FFF and FBF filters(FIG. 3) as those of the equalizer EQU. This results in the predistorteroperating with the same coefficients as those of the equalizer EQUduring a transmission time interval defined with respect to thereception time interval with a delay equal to that defined in the TimeDuplex mode of the DECT.

[0040]FIG. 5 shows a second embodiment of the invention. In thisembodiment, the output of the discriminator 221 is applied to ananalogic/digital converter A/D input. This converter samples the signalreceived from the remote unit with a higher sample frequency than thefrequency of the logic symbols associated to the received signal. Theselogic symbols are the bits to which the modulation/demodulation isapplied. Hence, sets of N samples are defined {φ(k, T)}, where N is aninteger, delimited by respective windows offset by a pitch of M samples,where M for example is equal to 1. A set of samples is defined by a samesample phase (between 0 and 2π) of the received signal. In the notation{φ(k, T)}, k indicates the range of the first sample of the set and Tindicates the bit period separating two consecutive samples of this set.A correlator COR correlates a sequence {l} with each one of the obtainedsample sets, to define an a priori optimum set of samples {φ(k=K, T)},defining a maximum correlation value. The sequence {l}, stored forexample in a ROM memory, includes samples at bit frequency of apredefined sequence received through the transmission chain of theremote unit and the reception chain of the base station. Hence, thesequence {l} is representative of the non-linear distortion of themodulation/demodulation means distributed between the base station andthe remote terminal. The predefined sequence is typically asynchronization sequence transmitted in each DECT raster.

[0041] To an input of the equalizer EQU, the a priori optimum sample set{φ(k, T)} is applied and also some {φ(K−2, T)}, {φ(K=1, T)}, {φ(k, T},{φ(K+1, T} and {φ(K+2, T)} of the other sample sets which are defined byrespective sample phases near to the sample phase of the a priorioptimum sample set. The equalizer EQU receives, on the one hand, a logicsymbol sequence TS stored in a ROM memory and which corresponds in theDECT to the training sequence and, on the other hand, the sample sets{φ(K−2, T)}, {φ(K−1, T)}, {φ(k, T)}, {φ(K+1, T)} and {φ(K+2, T)}.

[0042] The equalizer EQU produces an error signal ε(K−2), ε(K−1), ε(K),ε(K+1) and ε(K+2) for each one of the sample sets {φ(K−2, T)}, {φ(K−1,T)}, {φ(k, T)}, {φ(K+1, T)} and {φ(K+2, T)}. A selector chooses thesample phase for which the equalization error signal ε(K−2), ε(K−1),ε(K), ε(K+1) and ε(K+2) is smallest. Hence, an optimum phase Ksel forthe sample of the signal received is obtained.

[0043] After this synchronisation arrangement phase, a calculationmodule in the equalizer EQU defines the equalizer coefficientsminimizing the error signal according to, on the one hand, said signalreceived from the remote unit when sampled with the sample phase of theselected sample set and, on the other, a training sequence TS.

[0044] These coefficients are transmitted to the predistorter topredistort the signal to be transmitted.

[0045] In another possible alternative to this second embodiment, aninterpolation algorithm to define the sample sets ({φ(K−2, T)}, {φ(K−1,T)}, {φ(K+1, T)} and {φ(K+2, T)}) which are defined by respective samplephases near to the sample phase of the a priori optimum sample set canbe used. This solution permits the reduction of the sample frequency ofthe A/D converter.

[0046] In the DECT system, for the embodiments described above, thecoefficients used in the equalization means to equalize the signalduring a reception time interval are transmitted to the predistorter topredistort the signal during the following transmission time interval.In DECT, the ascending raster successively includes a synchronizationsequence, a training sequence and the transmission and receptionchannels in-temporary-duplex mode.

[0047] In order to improve the quality of the predistorted signal, aspectral control can be applied to the predistorted signal. Thisspectral control may be in the form of an automatic gain control, whichis applied to the predistorted signal imposing upon it a constantenvelope.

[0048] Another way to improve the quality of the predistorted signalconsists of studying the convenience of predistorting or not. Thepredistorter is activated when the non-linearity degree of the receivedsignal is less than a reference level. On the other hand, thepredistorter is not activated when the non-linearity degree of thesignal received is greater than this reference level.

1. Data transmission/reception unit to exchange data with a remote unitthrough a channel using means for modulation/demodulation treatmentdistributed between said data transmission/reception unit and saidremote unit, said transmission/reception unit including equalizationmeans (23) to equalize a first signal received from said remote unit andpredistortion means (13; 13 a, 13 b) to predistort a second signaltransmitted to said remote unit, characterized in that (a)—the means formodulation/demodulation treatment operate according to a non-linear modeand (b)—the unit includes (b1)—means (230, 231; l) to store informationwhich is representative of a non-linear distortion of saidmodulation/demodulation treatment means and which defines a non-linearreference, and (b2)—means (233; COR, EQU) to adjust coefficients (CO) inthe equalization means (23) according to the signal received from theremote unit and said representative information, said adjustedcoefficients being transmitted to said predistortion means (13; 13 a, 13b).
 2. Transmission/reception unit according to claim 1 , characterizedin that the data transmission/reception unit exchanges data with saidremote unit using a duplex mode by time division, and that said adjustedcoefficients (CO) are used in the equalization means to equalize saidfirst signal during a reception time interval and are transmitted tosaid predistortion means to predistort said second signal during thefollowing transmission time interval.
 3. Transmission/reception unitaccording to claim 1 , characterized in that the adjustment means arecalculation means (233) which minimize an error signal (ES) in theequalization means (233) according to, on the one hand, said signalreceived from the remote unit and, on the other hand, saidrepresentative information.
 4. Transmission/reception unit according toclaim 2 , characterized in that the means for non-linearmodulation/demodulation treatment are frequency modulation/demodulationtreatment means.
 5. Transmission/reception unit according to claim 3 ,characterized in that said unit is a base station of a radiocommunications system with the mobile phones according to the DECTstandard.
 6. Transmission/reception unit according to claim 1 ,characterized in that the adjustment means include: means (A/D) tosample said first signal received from the remote unit to a higherfrequency than a frecuency of logic symbols associated to the signalreceived and to define sample sets ({φ(k, T)}), each one defined by asample phase respectively, means (COR) to correlate said representativeinformation ({l}) to each one of the sample sets ({φ(k, T)}) of thesignal received from the remote unit, to define the a priori optimumsample set ({φ(k, T)}), means to select a sample set between said apriori optimum sample set ({φ(k, T)}) and some of said sample sets({φ(K−2, T)}, {φ(K−1, T)}, {φ(K+1, T)} and {φ(K+2, T)}) defined bysample phases near to the sample phase of said a priori optimum sampleset, the selected set being the set producing the smallest equalizationerror signal with respect to a logic symbol sequence (TS), andcalculation means to minimize an error signal in the equalization means,according, on the one hand, to said signal received from the remote unitwhen it is sampled with the sample phase of the selected sample set and,on the other, to said logic symbol sequence (TS). 7.Transmission/reception unit according to claim 5 , characterized in thatthe means for non-linear modulation/demodulation treatment are means forfrequency modulation/demodulation treatment.
 8. Transmission/receptionunit according to claim 6 , characterized in that said unit is a basestation of a radio communications system with the mobile phonesaccording to the DECT standard.